Chromatograph mass spectrometers combining a chromatograph such as a gas chromatograph (GC) or a liquid chromatograph (LC) and a mass spectrometer such as a quadrupole mass spectrometer are widely used to perform qualitative or quantitative analyses of various components contained in a sample. When performing qualitative analyses of known components using a chromatograph mass spectrometer, an SIM measurement method is typically used, whereby only ions having one or a plurality of specific mass-to-charge ratios designated in advance are selectively and repeatedly detected.
That is, in a chromatograph mass spectrometer, an analysis is executed by setting the mass-to-charge ratio of a known target component so that it is subjected to SIM measurements, whereby the elapsed time of the intensity of ions having this mass-to-charge ratio is obtained, so this can be plotted to find an extracted ion chromatogram (also called a mass chromatogram) with respect to the target component. In order to perform a qualitative analysis, peaks appearing in the vicinity of the retention times of known components are detected in this extracted ion chromatogram, and the peak areas are found and reflected in a predetermined calibration curve to reduce the areas to the content of the target component.
In order to perform a qualitative analysis using SIM measurements as described above, it is necessary to set analysis conditions such as the mass-to-charge ratios to be measured prior to analysis. For example, the chromatograph mass spectrometer described in Patent Literature 1 is provided with a function which, when an analyst creates a compound table describing information about compounds to be measured, automatically creates an analysis condition table based on the information described in the table. Such an analysis condition table creation function in a conventional chromatograph mass spectrometer will be described using a specific example.
FIG. 9 is an example of a compound table. As shown in the figure, the compound table includes information such as the compound name, the mass-to-charge ratio of a quantitative ion, the mass-to-charge ratio of a confirmation ion, the predicted retention time, and the measurement time range for each compound. A qualitative ion is an ion which best characterizes the compound. A confirmation ion is an ion having a mass-to-charge ratio differing from that of the qualitative ion which characterizes the compound. This confirmation ion is used to confirm that the chromatogram peak of the qualitative ion is derived from the target compound using the relative ratio of the signal intensity of the confirmation ion peak and the signal intensity of the qualitative ion peak in the mass spectrum. The predicted retention time is the predicted value of the time of elution from a column in the chromatograph. The measurement time range is a parameter for designating the time range for which the compound should be measured around the predicted retention time.
FIG. 10 is an example of an analysis condition table created automatically with respect to the compound table described above. In the analysis condition table, the analysis conditions for one compound are summarized in one line as a “measurement event.” That is, each measurement event includes the compound name as well as the measurement mode (selection of the SIM measurement mode or the scan measurement mode), the mass-to-charge ratios of measurement ions, the measurement start time, the measurement end time, and the event time. The mass-to-charge ratios of the qualitative ion and the confirmation ion of the compound to be measured are set for the mass-to-charge ratios of the measurement ions. A time determined by tracing back by the measurement time range from the predicted retention time of the compound to be measured is set for the measurement start time. The time after the measurement time range has elapsed from the predicted retention time of the compound to be measured is set for the measurement end time. The event time is the unit time of the repetition of the measurement event, and this is set to a value predetermined by the analyst.
In FIG. 10, the measurement event #1 (the measurement event number is expressed by “#”) for measuring compound A, for example, is in the SIM measurement mode, which means that two mass-to-charge ratios of m/z=100 and m/z=200 are repeatedly measured in 100 msec units for 2 minutes from the 9.0 min mark to the 11.0 min mark. As a result of executing an analysis in accordance with the analysis condition table shown in FIG. 10, an extracted ion chromatogram such as that shown in FIG. 6, for example, is obtained. However, although only one chromatogram is shown for one measurement event here, the number of chromatograms created corresponds to the number of measurement ions (in this example, two for each compound).
As can be seen from FIGS. 6 and 10, the three measurement periods of the measurement events #1, #2, and #3 overlap during the 0.8 minutes from the 10.2 min mark to the 11.0 min mark. Since a plurality of measurements with overlapping measurement periods will be performed sequentially in a time-sharing manner, the measurement point time interval for one compound becomes wider as the number of overlapping measurement periods increases. For example, since three measurement events overlap in the period from the 10.2 min mark to the 11.0 min mark, an event time of 100 msec×3=300 msec for one measurement becomes the measurement point time interval. Here, the measurement point time interval is called the loop time. In the example shown in FIG. 6, the loop times in the measurement sections <1> to <7> are as shown in FIG. 11.
When the predicted retention times of a plurality of compounds to be measured are in close proximity to one another, the overlapping of different measurement events in time increases, so the loop times increase. When the loop time increases in a given measurement section, the measurement point time interval opens in that measurement section, which reduces the number of data points constituting the chromatogram peak and reduces the precision or reproducibility of the peak area. As a result, the precision or reproducibility of the quantitative analysis is reduced. In order to obtain sufficient peak area reproducibility, at least ten data points are typically necessary for one chromatogram peak, and it is necessary to increase the number of data points further depending on the required quantitative precision or reproducibility. Therefore, the analyst determines the upper limit of the loop time for each compound in accordance with the required quantitative precision or reproducibility and adjusts the parameters of the measurement event so that the loop time determined by the analysis conditions is equal to or less than the upper limit.
Specifically, after an analysis condition table is automatically created based on the compound table as described above, the analyst confirms whether the loop time of each measurement interval is equal to or less than the predetermined upper limit. If the loop time exceeds the upper limit, the analyst adjusts the start time or the end time of the measurement event so that there is no overlapping of measurement events. However, in a conventional chromatograph mass spectrometer, such an assessment of the overlapping of measurement sections or the parameter adjustment of measurement events must be performed manually by the analyst in the analysis condition table or the like, which is a very troublesome and time-consuming operation for the operator. Moreover, there is also a risk of setting parameters erroneously, in which case appropriate quantitative results cannot be obtained.